Allah is Aware of All Things
Ibn `Abbas said, "They used to dislike facing the sky with their private parts, particularly during sexual relations. Therefore, Allah revealed this verse." Al-Bukhari recorded by way of Ibn Jurayj, who reported from Muhammad bin `Abbad bin Ja`far who said, "Ibn `Abbas recited, (أَلَا إِنَّهُمْ تَثْنَونِي صُدُورُهُمْ) "Behold their breasts did fold up." So I said: `O Abu Al-`Abbas! What does -their breasts did fold up- mean' He said, `The man used to have sex with his woman, but he would be shy, or he used to have answering the call of nature (in an open space) but, he would be shy. Therefore, this verse,
أَلا إِنَّهُمْ يَثْنُونَ صُدُورَهُمْ
(No doubt! They did fold up their breasts,) was revealed."' In another wording of this narration, Ibn `Abbas said, "There were people who used to be shy to remove their clothes while answering the call of nature in an open space and thus be naked exposed to the sky. They were also ashamed of having sexual relations with their women due to fear of being exposed towards the sky. Thus, this was revealed concerning them." Al-Bukhari reported that Ibn `Abbas said that
يَسْتَغْشُونَ
(they cover themselves) means that they cover their heads.
Tafsir Ibn Kathir (abridged, English).